The impregnation of absorbent materials, also called carrier matrices, is a common process which is frequently used, in particular, in the production of test strips. For example, test strips for diagnostic detection of glucose can be produced by impregnation absorbent paper first with the organic solution of a chromogen (for example 3,3',5,5'-tetramethylbenzidine in acetone) and then with an aqueous buffered enzyme solution (glucose oxidase or peroxidase) and drying the paper. The impregnated papers are then attached to carrier films, which function as the test strip holder.
The impregnating processes are usually carried out by an immersion method. Here, the absorbent substrate to be impregnated is conveyed at a constant rate through an immersion dish containing the impregnating solution to be impregnated and is then dried.
A serious disadvantage of this process is that during impregnation with multi-component impregnating solution systems, increasing concentration gradients develop in the impregnating solution or in the carrier matrix as the duration of the impregnation increases, since the various components are as a rule absorbed to different degrees by the matrix. The quality of the test strips and hence the accuracy of the analytical results obtained with the test strips is thereby impaired. Furthermore, precise metering of the amount of liquid to be impregnated is not possible with this process. The amount of liquid absorbed is rather determined by the absorbency of the carrier matrix.
Several successive impregnations of the same carrier matrix also present problems in the immersion impregnation process, since the components of the reagents impregnated beforehand can be extracted again by the subsequent impregnation, especially if the impregnations are carried out from the same solvent.
Another process for impregnation of absorbent substrates is the spraying process. Here, the impregnation liquid is sprayed from spray guns onto the continuously moving matrix material and the matrix material is then dried. Although the above- o mentioned disadvantages of the immersion process can be prevented here, this method is generally limited to liquids of low viscosity, which means that the field of application is restricted.
Impregnation of narrow, very sharply defined zones on a matrix also presents problems in the spraying process.
Test strips for the diagnostic field have been produced by a procedure in which the matrices, impregnated with the corresponding detection reagents, are cut into narrow strips and attached to polymer films or substrates, which function as the test strip holder. In addition to various adhesives used, which frequently have an adverse influence on the functioning of the detection reagents, the build-up of the test strips is also a disadvantage in carrying out the detection reaction. Thus, when blood is applied and is wiped off the reagent field after a defined residence time, complications arise on the one hand due to residues of blood and on the other hand the cottonball used to wipe off the blood remains stuck to the edges between the test strip holder and the reagent zone. In the case of urine test strips, which as a rule contain several different reagent zones (for example a glucose, pH, ketone, bilirubin, nitrite and hemoglobin zone) on one test strip holder, there are complications with the conventional systems in that residues of liquid remain between the attached reagent zones after immersion in the urine. In both cases, as well as aesthetic disadvantages, errors with respect to the accuracy of the test results frequently occur.